The injection of antigen into an animal results in the production of antibodies which specifically react with the immunizing antigen. Contained within the specificity of these antibodies are new antigens called idiotypes. Anti-idiotypic antibodies can be raised by immunizing animals with the original antibodies (idiotypes). An anti-idiotypic antisera was used to demonstrate crossreactivity between surface T cell receptors for soluble antigens (OVA) and a hybridoma anti-OVA antibody. Two other hybridoma anti-ovalbumin antibodies are being used as immunogens in goats in order to product additional anti-idiotypic sera. Also, splenic T cells from mice primed with blast T cells with anti-ovalbumin specificity are responsive in culture to mouse serum anti-ovalbumin. Other findings with rosette assays indicate that both B and T cell subpopulations from mouse spleens can bind ovalbumin, as well as can the hybridoma cell line. Anti-Ig binding rosettes are not observed with hybridoma cells although the secreted product is antibody and forms reverse plaques. Proliferation assays show a loss in responsiveness to specific antigens when unfractionated lymph node cells are tested, although fractionated "T" lymph node cells retain their proliferative function.